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COMPONENTS IN THE S YSTEM UNIT
System Unit
The system unit is a box-like case (usually rectangular in shape) that houses the electronic components of the computer.
For a desktop computer , the electronic components and most storage devices such as floppy drive, hard disk, and CD-ROM drive, reside inside the system unit. Other devices, known as the peripherals , such as keyboard, mouse, monitor, and printer, are connected externally to the system unit.
For a laptop computer , the system unit houses almost all of its components including the keyboard, pointing device, and monitor.
The Motherboard
Many of the electronic components in the system unit reside on a circuit board called the motherboard or system board. Several different types of chips can be found on the motherboard.
A chip is a small piece of semi-conducting material, on which one or more integrated circuits are etched. ¾ An integrated circuit (IC) is a microscopic pathway capable of carrying electrical current. Each integrated circuit can contain millions of transistors , which act as electronic switches, or gates, that open or close the circuit for electronic signals.
The central processing unit is one of the more important chips on the motherboard.
Central Processing Unit
The central processing unit (CPU) or processor interprets and carries out the instructions that operate a computer.
On larger computers , the various functions performed by the CPU are spread across many separate chips and sometimes multiple circuit boards.
On a personal computer , the CPU usually is contained on a single chip and is often called a microprocessor. A microprocessor usually contains the control unit , the arithmetic and logic unit , registers , and system clock.
The control unit directs and coordinates most of the operations in the computer. ¾ For every instruction, the control unit repeats a set of four basic operations called the machine cycle or instruction cycle. Fetching is the process of obtaining a program instruction or data item from memory. 9 The time taken to fetch is called instruction time , or I-time. Decoding is the process of translating the instruction into commands that the computer understands. Executing is the process of carrying out the commands. 9 The time taken to decode and execute is called execution time , or E-time. Storing is the process of writing the result to memory. ¾ Most processors used by personal computers today support pipelining , which means that the CPU begins executing a second instruction before the first instruction is completed. The result is faster processing. Most newer processors can pipeline up to four instructions. Superscalar CPUs have two or more pipelines that can process instructions simultaneously. ¾ A computer’s speed is very often measured according to the number of instructions it can process in one seconds. This speed is measured in MIPS , which stands for million instructions per second. 9 MIPS refers only to the CPU speed, whereas applications generally are limited by other factors such as input and output speed.
The arithmetic and logic unit (ALU) performs the execution part of a machine cycle. ¾ Specifically, the ALU performs the arithmetic , comparison , and logical operations. Arithmetic operations include addition, subtraction, multiplication, and division. Comparison operations involve comparing one data item to another to determine if the first item is greater than, equal to, or less than the other item. Logical operations work with conditions and logical operators such as AND, OR, and NOT.
9 The clock speed affects only the CPU; it has no effect on peripherals such as a printer or disk drive. Clock speed is an important consideration when buying a computer. Recently, copper has replaced aluminum to create the electronic circuitry of CPUs because
Copper is cheaper and also a better conductor of electricity. ¾ Processor chips that use copper run faster and cost less than chips that use aluminum.
Copper requires less electricity. ¾ More suitable for use in portable computers. Integrated CPU is a new type of microprocessor that combines functions of a CPU, memory, and a graphics card on a single chip.
These chips are designed for lower-cost personal computers and smaller-sized computers. Most of today’s processors are equipped with MMXTM^ technology , in which a set of instructions is built into the processor so that it can manipulate and process multimedia data more efficiently. Most of today’s computers are equipped with a zero-insertion force (ZIG) socket , which has a small lever or screw designed to facilitate the installation and removal of processor chips. Newer processor chips generate a lot of heat and a heat sink is a small ceramic or metal component with fins on its surface that is designed to absorb and ventilate heat produced by electrical components.
A smaller device called a heat pipe is used to cool laptop computers. A coprocessor is a special processor chip or circuit board designed to assist the processor in performing specific tasks.
Users running engineering, scientific, or graphics applications will notice a dramatic increase in speed with a floating-point coprocessor. ¾ A floating-point coprocessor is also called a math or numeric coprocessor. Some computers support parallel processing , which can speed up processing time by using multiple processors simultaneously to execute a program.
Parallel processing requires special software designed to recognize how to divide up the problem and then bring the results back together again.
Data Representation
Most computers are digital , meaning that they understand only two discrete states: on and off.
Computers are electronic devices powered by electricity, which has only two states: on, or off.
0 is used to represent off , and 1 is used to represent on.
In contrast, human speech is analog , meaning that it uses continuous signals to represent data and information. Computers use the binary system that has just two digits, 0 and 1.
Each of these digital values is called a bit , the abbreviation form for binary digit.
A bit is the smallest unit of data the computer can handle. Practically, computers process data in terms of bytes.
A bit is not very informative because it can only represent a value of either 0 or 1.
A byte is eight bits grouped together as a unit, and it provides enough different combinations of 0s and 1s to represent 2^8 = 256 individual characters that include numbers, letters, and symbols.
A byte is actually a series of on/off electrical states. A coding scheme is the patterns of 0s and 1s combinations used to represent characters.
The three most popular coding schemes used to represent data are: ¾ ASCII (American Standard Code for Information Interchange, pronounced as ASK-ee). ASCII is the most widely used coding system to represent data. ASCII is used on many personal computers and minicomputers. ¾ EBCDIC (Extended Binary Coded Decimal Interchange Code, pronounced as EB-see-dic). EBCDIC is used primarily on mainframe computers. ¾ Unicode Unicode is a coding scheme capable of representing all the world’s current languages. 9 ASCII and EBCDIC codes are sufficient for English and Western European languages, but are not large enough for Asian and other languages that use different alphabets. 9 Unicode is a 16-bit code that has the capacity of representing more than 65500 characters. 9 The first 256 codes of Unicode are the same as ASCII.
Volatile Memory
RAM (Random Access Memory) is the memory chips in the system unit. RAM is an example of volatile memory.
Items in RAM that are needed for future use must be saved to a storage device before the computer is turned off. There are two basic types of RAM, dynamic RAM and static RAM.
DRAM (Dynamic RAM) is a type of memory that must be re-energized constantly or its contents will be lost. ¾ SDRAM (Synchronous DRAM) is a faster version of DRAM because it is synchronized to the system clock. ¾ RDRAM (Rambus DRAM) is a newer type of DRAM that is even faster than SDRAM. ¾ Most computers today use some type of SDRAM or RDRAM.
SRAM (Static RAM) is faster and more reliable than any form of DRAM, but are more expensive. ¾ SRAM does not have to be re-energized as often as DRAM. RAM chips are usually packaged on a small circuit board that is inserted into the motherboard.
SIMM (Single Inline Memory Module) refers to the type that pins on opposite sides of the circuit board are connected together to form a single set of contacts.
DIMM (Dual Inline Memory Module) refers to the type that pins on opposite sides of the circuit board are not connected and thus form two sets of contacts.
RAM chips commonly used in SIMMs and DIMMs are SDRAM chips.
RDRAM chips are packaged on a circuit board called RIMM (Rambus Inline Memory Module). The necessary amount of RAM varies according to the types of work to do and the types of applications to be used on the computer. Most of the computers today use memory cache. Memory cache speeds the processes of computing by storing frequently used instructions and data.
There are two types of memory cache: ¾ L1 (Level 1) cache is also called the primary cache or internal cache. L1 cache is built directly into the processor chip (usually ranges from 8 KB to 64 KB).
¾ L2 (Level 2) cache , or external cache , is not part of the processor chip. L2 cache is slightly slower than L1 cache, but has a larger capacity (usually ranges from 64 KB to 2 MB). A computer system with L2 cache usually performs at speeds that are 10 to 40 percent faster than those without any cache. A computer system should have from 256 K to 512 K of L2 cache to realize the largest increase in performance. 9 The increase in performance is not significant above 512 K.
When the processor needs an instruction or data, it searches memory in the following order:
- L1 cache
- L2 cache
- RAM
- Storage device (e.g., hard disk, floppy disk, CD-ROM)
Nonvolatile Memory
ROM (Read Only Memory) is the name given to memory chips storing data that can only be read.
ROM is an example of nonvolatile memory.
ROM contains data, instructions, or information that is recorded permanently when the chip is manufactured. ¾ ROM chips that contain permanently written data, instructions, or information are called firmware. A programmable read-only memory (PROM) is a type of ROM chip on which permanently items can be placed.
The instructions used to program a PROM chip are called microcode.
Once a PROM chip is written, it functions like a regular ROM chip and cannot be erased or changed. Flash memory or Flash ROM can be erased electronically and reprogrammed.
Flash memory is used to store programs and data on personal computers, cellular phones, printers, digital cameras, pagers, and personal digital assistants (PDA). CMOS (Complementary metal-oxide Semiconductor) is used to store configuration information about the computer.
CMOS chips use battery power to retain information even when the computer is turned off.
Unlike ROM, information stored in CMOS can be changed.
The height or thickness of PC Card various among three types ¾ The thinnest Type I cards are used to add memory capabilities to the computer. ¾ Type II cards contain communications devices such as modems. ¾ The thickest Type III cards are used to house devices such as hard disks. The ability to add or remove devices while a computer is running is called hot plugging or hot swapping.
Ports
External devices are often attached by a cable to the system unit. A port is the interface, or point of attachment, to the system unit. Ports have different types of connectors.
A connector is used to join a cable to a device. ¾ Male connectors have one or more exposed pins. ¾ Female connectors have matching holes to accept the pins on a male connector.
A gender changer is a device used to join two connectors that are either both female or both male. A serial port transmits only one bit of data at a time.
Serial ports are usually used to connect devices that do not require fast data transmission rates, such as a mouse, keyboard, or modem. ¾ A modem uses a serial port because the telephone line expects data to be transmitted in a serial form.
Serial ports conform either to the RS-232 or RS-422 standard, which specifies the number of pins used on the port’s connector. ¾ Two commonly used connectors for serial ports are a male 25-pin connector or a male 9-pin connector. A parallel port is capable of transferring more than one bit of data at a time.
Parallel ports are usually used to connect printers and scanners. ¾ Many printers connect to the system unit using a parallel port that can transfer eight bits of data simultaneously through eight separate lines in a single cable with a 25-pin female connector.
A parallel port is sometimes called a Centronics interface , after the company first defined the standard for communication.
EPP (Enhanced Parallel Port) and ECP (Extended Capabilities Port) use the same connectors as the Centronics port, but are more than ten times faster.
A USB (Universal Serial Bus) port can connect up to 127 different peripheral devices with a single connector.
USB also supports hot plugging as well as Plug and Play.
Devices connect to the system unit using USB ports include keyboard, mouse, scanner, disk drives, digital camera, … A MIDI (musical instrument digital interface) port is designed to connect the system unit to a musical instrument, such as an electronic keyboard.
MIDI is a standard of the electronic music industry.
A system unit with a MIDI port has the capability of recording sounds that have been created by a synthesizer and then processing the sounds to create new sounds. ¾ A synthesizer is a peripheral or a chip that creates sound from digital instructions. A SCSI (small computer system interface) port is a special high-speed parallel port used to attach peripheral devices such as printers and disk drives to the system unit.
A SCSI port can transmit up to 32 bits of data at a time.
A total of seven SCSI devices can be daisy chained together. The 1394 port , also called FireWire , can connect multiple types of devices requiring faster data transmission speeds.
Devices that normally require 1394 port for connection include digital video camcorders, digital VCRs, color printers, scanners, digital cameras, …
The 1394 port also supports Plug and Play. An IrDA (Infrared Data Association) port allows wireless devices to transmit data via infrared light waves.
The IrDA port on the computer and the IrDA port on the peripheral device must be aligned so that nothing obstructs the path of the infrared light wave.
Devices that use IrDA ports include keyboards, mouse, printer, …
Buses
A bus is an electrical channel that allows the various devices inside and attached to the system unit to communicate with each other. All buses consist of two parts: a data bus and an address bus.
The data bus transfers actual data, and
The address bus transfers information about where the data should go in memory.
The USB bus and 1394 bus eliminate the need to install expansion cards into expansion slots. 9 USB devices are connected to each other outside the system unit. The USB port then connects to the USB, which connects to the PCI bus on the motherboard. 9 The 1394 bus works in a similar way as the USB bus. The PC Card bus is the expansion bus for a PC Card. 9 Data travels on the PC Card bus to the PCI bus.
Bays
A bay is an open area inside the system unit used to install additional equipment.
A bay is different from a slot, which is used for the installation of expansion cards.
Bays are most often used for disk drives. ¾ Drive bays are bays used for disk drives. An external drive bay or exposed drive bay allows access to the drive from outside the system unit. 9 Floppy disk drives, CD-ROM drives, and tape drives are examples of devices installed in external drive bays. An internal drive bay or hidden drive bay is concealed entirely within the system unit. 9 Hard disk drives are installed in internal bays.
Power Supply
The power supply is the component in the system unit that converts the wall outlet AC power into DC power. Some external peripheral devices also have an AC adapter , which is an external power supply.
Laptop Computers
A docking station is a device into which a laptop computer is placed. A docking station contains connections to peripherals such as keyboard, monitor, printer, and other devices. Some laptop computers have a port replicator , which is a device that allows other peripheral devices such as printer, modem, and mouse to connect into it. The port replicator is then connected to the laptop computer.
